1. Field of the Invention
This invention relates to the production of low molecular weight polyanhydrides by the mass polymerization of maleic monomers and alkyl styrenes.
2. Discussion of the Prior Art
The production of low molecular weight polyanhydrides from maleic monomers and alkyl styrenes is disclosed, for example, in U.S. Pat. Nos. 3,732,332 (1973), 3,789,038 (1974), and 3,929,738 (1975) to Curtis et al. For example, it has been proposed to produce such polyanhydrides by the polymerization, in the absence of a catalyst, of a maleic monomer and at least one alkyl-substituted styrene. More specifically, the above patents disclose the production of low molecular weight polyanhydrides having softening points within the range of about 111.degree. to 156.degree. C. by the mass polymerization reaction, in the absence of catalysts, of maleic anhydride and alpha-methyl styrene in a mol ratio of maleic monomer to alpha-methyl styrene of greater than 1:1, by heating at least a major portion of the maleic anhydride to a temperature of about 160.degree. to 200.degree. C., adding the alpha-methyl styrene to the maleic anhydride with agitation and continuing the polymerization until essentially all of the maleic anhydride and alpha-methyl styrene have polymerized. Such patents disclose that ctalysts, such as tertiary butyl perbenzoate, are not necessary or desirable.
The resultant polyanhydrides, such as the dianhydride of maleic anhydride and alpha-methyl styrene, are useful, for example, as hardeners for epoxy molding compounds. Prepolymers of such polyanhydrides with epoxy resins are also useful as hardeners for epoxy molding compounds.
However, it has been found that in the production of such polyanhydrides, a considerable amount of undesirable monomer and other materials is present in the polymerized mixture and since such materials have been found to interfere with the desired hardening reaction in epoxy molding compounds, in many cases the polyanhydrides must be vacuum stripped to remove such materials. As an example, in the production of such polyanhydrides, it is often necessary to vacuum strip the reaction products at a temperature in the range of about 160.degree. to 200.degree. C. under a vacuum of about 26 to 30 inches of mercury for about 21/2 hours. As a result of such vacuum stripping, typically about 5 to 15 weight percent of monomeric and other undesirable materials are recovered from the polyanhydride.
It would be desirable if a process were provided by which a more effective conversion of the maleic monomer and alphamethyl styrene were effected such that the resultant product need not be subjected to vacuum stripping or, if the vacuum stripping is employed, under less severe conditions.